This research is part of a general program to measure the electronic structure of biologically important molecules in those instances where there is good evidence that the electronic structure is essential to the biological function. The present work is concerned with polyene photoisomerization in the visual pigment and vitamin D 3. In order to understand the process of vision on a microscopic scale it is necessary to determine the nature of the interactions between opsin and retinal, both before and after the absorption of a photon. This description in turn rests on a detailed description of the electronic states of the visual pigments and their behavior under perturbation. This description is to be derived from measurements of the optical absorption and emission spectra of model systems and rhodopsin. A similar situation holds for vitamin D3 and its precursors. The emphasis is on developing new information by exploiting recent advances in polyene electronic structure and the application of tunable lasers to solve spectroscopic problems. This proposal describes experiments aimed at 1) Determining those 1-photon and 2-photon spectra for hexatriene and butadiene that are needed to establish the relative energies and conformations of the low lying excited singlet states, especially the 2(1)A(g) state. 2) Measuring fully resolved spectra for retinal derivatives seeded in supersonic helium expansions. 3) Elaborating the mechanism by which the model polyene octatetraene photoisomerizes in a condensed phase molecular cavity. 4) Extending these high resolution spectroscopic analyses to rhodopsin through measurements of photochemical hole burning.